Help me seal my gas cap!

The previous owner of my Commando put a beautiful lacquer black finish on the gas tank. He did not clear coat it.
Yesterday, I filled the tank to about an inch from the top for a ride on a fine autumn day.
I remember going over some rough railroad tracks which could have thrown the gas around, I don't know.
But when I got home there were streaks etched into the finish where gas leaked out over the tank. Needless to say, I was bummed out.
I was able to rub about 90% of them out with polishing compound but now the tank has swirl marks.
How can I seal that cap so this won't happen again? It's the original cap, the snap closed variety. Thanks in advance for any help.

That's the down side of lacquer - soft & susceptible to most every solvent out there. You might try Meguiar's polish to see if it'll buff out the swirl marks, it's a little finer than most, I think.

You probably need a new gas cap gasket - Old Britts has them for sure, prolly a lot of other suppliers that I don't know about. Old Britts item number is 060681/G.
HTH,
Ted

My tank cap started to leak out of the blue earlier this year, and all I did to cure the leak was make sure that there was no debris or paint "bits" that had adhered to the seal/gasket in the cap, and i used 1000 grade wet and dry on the filler neck as the filler neck was not perfectly smooth, and cured the leak somehow. I still don't really know why it started.

As TC said, a new gasket may not go amiss.

I fought this problem on a steel Interstate tank a few years ago. I replaced the cap with a new one, carefully filed the top of the tank opening flat. Still leaked. It turned out the filler neck leaked where it was brazed into the tank. Whenever the tank was more than 2/3 full it would seep out during a ride.

I finally cleaned and sealed the tank (there were a couple rusty spots inside). When I sealed it a left it upside down so any excess would pool around the filler neck.

It worked.

Seems like I had one leak on an old fibreglass Commando tank. Gaskets were hard to comeby so I treated it with aircraft rubber protectant (good stuff) and put an alloy shim under it.
Since gaskets are easily available now that would probably be only for emergencies.

If the gasket is in good nick gently tap the tank neck latching lip down a little and you'll find it will pull the seal down on to its seat.

Worked for me

Cash